Apparatus utilized for the detection of neutrons can be divided into two major categories: detectors for slowly traveling neutrons and detectors for rapidly traveling neutrons.
In general, the detectors are adapted to a range of energy and one cannot use them interchangeably for the measure of neutrons from one or the other category.
The slow neutron detectors comprise ionization chambers and proportional counters. These detectors contain boron (.sup.10 B) either in the form of deposits on the walls or the electrodes of the detectors or in gaseous form in the case of counters of boron trifluoride (BF.sub.3).
With this type of detector one counts the alpha particles (4.sub.H e) emitted at the moment of interaction of a thermal neutron with a boron atom (.sup.10 B).
In the presence of a large amount of gamma noise, it is preferable to utilize the phenomena of fission of U.sup.235 or of Pu.sup.239 to be able to ionize the fission fragments (greater than those of alpha particles).
The detectors activated by slow neutrons are constructed from elements in the form of sheets which have large areas for interaction (neutron, gamma). The essential interest of this type of detector resides in its insensitivity to gamma rays as well as in its simple construction. Its major disadvantage is in not emitting a direct and continuous reading.
Included in the category of slow neutron detectors are scintillation counters obtained by the association of scintillators of very high efficiency and photomultipliers.
Included in the category of rapid neutron detectors are elastic diffusion detectors, exo-energetic reaction detectors threshold detectors and armored detectors.
The elastic diffusion detector utilizes essentially diffusion (neutron, proton) in a hydrogen medium.
The exo-energetic reaction detector utilizes lithium (.sup.6 Li) in scintillators or the type LiI (T1) associated with a photomultiplier and in the form of thin layers disposed between two semi-conductors of silicon.
With this type of reaction one can also utilize a rare gas (.sup.3 He) either in a proportional counter or in a narrow chamber provided between two detectors of charged particles.
The relatively high values of Q of these reactions permit good discrimination from the background gamma noise.
The detectors for threshold reactions only act on neutrons having an energy greater than a certain value. These reactions are of three orders: fission, reaction (n,p) or (n,.alpha.) or (n,2n), and non-elastic diffusions.
Armored detectors utilize detectors of the type previously described for the detection of slow or thermal neutrons and they are completely surrounded by a moderator material for measuring rapid neutrons.
It is also necessary to underline that the neutron counters which have just been discussed are generally specific in the sense that they are poorly adapted to the counting of other rays (alpha, beta, gamma).
The technological examination of nuclear detectors which has been previously described demonstrates that there does not exist at the present time portable detectors of small size which require for their operation low electrical voltages while furnishing an immediate response for any type of ray.
In fact, apparatus for radio protection are always constituted either of large assemblies associated with scintillator probes or Geiger Muller tubes very slightly sensitive to gamma rays and insensitive to neutrons (except for special tubes).
The first type of apparatus necessitates high voltages of the order of several hundreds of volts as well as operation with different scintillators for each type of particle. The second type do not have great efficiency.